Antibacterial photodynamic therapy (aPDT) is a treatment for bacterial infections. Systemic side effects may be greatly reduced and the bacteria specifically targeted without the immediate risk of inducing bacterial resistance (Hamblin M R, et al., Photochem Photobiol Sci 2004; 3: 436-450; Ikai H, et al., PloS one 2013; 8: e81316). aPDT utilizes a combination of visible light, a photosensitizer (PS) and oxygen to produce cytotoxic species.
Porphyrins are aromatic heterocyclic compounds that are ubiquitous in nature, and have been widely investigated in photodynamic therapy of tumours and microbial infections (Almeida A, et al., Compr Ser Photoch 2011; 11: 83-160; O'Connor A E, et al., Photochem Photobiol 2009; 85: 1053-1074; Mang T S, et al., Lasers Surg Med 2012; 44: 588-596; Berenbaum M, et al., Br J Cancer 1986; 54: 717; Nitzan Y, et al., Photochem Photobiol 1992; 55: 89-96; Karunakaran S C, et al., ACS Chem Biol 2013; 8: 127-132; Prasanth C S, et al., Photochem Photobiol 2014; 90: 628-640; Banfi S, et al., J Photochem Photobiol B: Biol 2006; 85: 28-38).
In cancer treatment neutral and negatively charged porphyrins (e.g., porfimer sodium) and the related chlorins (e.g. meso-tetrahydroxyphenylchlorin) have been studied as effective treatment modalities (O'Connor et al., supra). However, in aPDT there is a general agreement that positively charged groups on the porphyrin is important for antibacterial photodynamic effect (Prasanth et al., supra; Banfi et al., supra; Stojiljkovic I, et al., Expert Opin Invest Drugs 2001; 10: 309-320; Merchat M, et al., J Photochem Photobiol B: Biol 1996; 35: 149-157). This understanding reasons in the outer wall and cytoplasmic membrane structure of Gram-positive and Gram-negative bacteria. The Gram-positive bacteria outer wall consists of a porous peptidoglycan layer with traversing lipoteichoic acids where even 30 000-60 000 Da peptides may diffuse through (Friedrich C L, et al., Antimicrob Agents Chemother 2000; 44: 2086-2092). The Gram-negative bacteria outer structure contains an additional highly organized outer lipid bilayer membrane composed of negatively charged lipopolysaccharides, polysaccharides, proteins and lipoproteins (Maisch T, et al., Photochem Photobiol Sci 2004; 3: 907). This structure usually renders the bacteria resistant to negatively charged or neutral porphyrins which efficiently photoinactivate Gram-positive bacteria (Hamblin et al., supra; Nitzan et al., supra; Maisch T, et al., supra; Malik Z, et al., J Photochem Photobiol B: Biol 1990; 5: 281-293).
Previous formulations of porphyrins have mostly been based on simple preparations in water, liposomes and DMSO (Banfi et al., supra; Merchat et al, supra; Tang H M, et al., J Inf Chemother 2007; 13: 87-91). Particularly for the neutral porphyrins, these preparations have been inefficient in photoinactivation of Gram-negative bacteria. In aPDT it is advantageous to use water miscible formulations as the PS needs to penetrate aqueous exudate or other body fluids to reach the target bacteria. Also the bacterial outer cover is largely hydrophilic and will not allow close interactions with hydrophobic compounds (Malik Z, et al., J Photochem Photobiol B: Biol 1992; 14: 262-266). A neutral, hydrophobic PS incorporated in liposomes will therefore not readily be released from the carrier and interact with the bacterial membrane (Merchat et al., supra; Haukvik T, et al., Pharmazie 2009; 64: 666-673).
Additional formulations are needed to aid in inactivation of bacteria and other microorganisms.